1. Field of the Invention
The invention relates to a method and an apparatus for producing optical glass elements, in particular optical prisms or optical rod lenses, using a drawing process.
2. Description of Related Art
Optical glass elements, such as for example prisms or lenses, can be obtained from a block of optical glass by cutting, grinding and then polishing. However, this represents a time-consuming and expensive process.
Additional problems arise if optical glass elements with a small cross section or small dimensions are being produced, since they are much more difficult to handle and sufficient accuracy of the edge sharpness, edge angle, flatness of the surfaces and a low roughness of the surfaces, which are crucial factors in determining the quality of an optical glass element, are only possible with considerable work and therefore also at high cost.
However, miniaturized optical glass elements, in particular microlenses and/or microprisms, are becoming increasingly important in photography, in particular digital photography in cell phones, and need to be inexpensive to mass produce. Further application areas for microprisms are beam splitters in optical pick-up systems. Cylindrical microlenses are also employed in the beam shaping of diode lasers as “fast axis collimator lenses (FACs)”.
One known method for producing optical glass elements with a small cross section is to use a drawing process. In this case, a generally continuous glass strand of a selected preform or a selected cross section is fed to a heating apparatus. The glass is heated until it is plastically deformable. A drawing apparatus draws the glass strand out of the heating apparatus, so as to form a draw bulb. The drawing operation reduces the cross section of the glass strand in a targeted way and then the glass strand is severed from the drawn glass strand, for example by cutting, to form a glass element of the desired size or length.
The main objective in this context is to reduce the glass strand to a desired cross section and to transfer the geometry of the preform to the end product or the final shape of the glass element to be produced as far as possible without changing this basic geometry. Significant parameters in this method include the rate at which the glass strand is fed into the heating apparatus, the size of the heating apparatus, the temperature selected in the heating apparatus to achieve an optimum viscosity of the glass strand and the draw rate of the glass strand out of the heating apparatus.
Although the drawing process transfers the basic geometry or contour of the preform to the glass element that is to be produced, deformation does occur, for example along the cross section of a glass strand a concave side face is formed from an originally planar side face, and consequently under certain circumstances the final shape of the glass element that is to be formed may differ significantly from the preform. This requires a further working step to correct this deformation by means of grinding and then polishing, which is highly complex in the case of the present optical components of small cross section, for the reasons mentioned above.
Conventional measures for minimizing these changes include, inter alia, the selection of a preform which corresponds to the desired final shape as accurately as possible, the selection of a deformation viscosity which is as high as possible, to ensure uniform heating of the glass strand the selection of a temperature which is constant over the course of time in the heating apparatus, the selection of a temperature which is as uniform as possible over the periphery of the glass strand, i.e. ideally a temperature distribution which is axially symmetrical along the longitudinal axis, and the introduction or positioning of the glass strand in the heating apparatus in such a manner that the glass strand is guided in the center of the heating apparatus and that the longitudinal axis of the glass strand lies as accurately as possible on the longitudinal axis of the heating apparatus and therefore on the axis of symmetry of the temperature field.
To improve the flatness and smoothness of the surfaces while at the same time ensuring similarity of the cross section of a drawn optical glass element with its parent glass, US 2002/0014092 A1 describes a method involving providing a parent glass which has a cross section that is substantially almost identical to the desired cross section of the optical glass element that is to be produced. This document does not give any indication that the cross section of the parent glass deliberately takes a different shape than the optical glass element to be produced. Moreover, in the case of a complex optical element, for example a polygonal prism, the provision or production of a parent glass which is substantially almost identical to the desired cross section of the optical glass element to be produced can entail considerable outlay in terms of labor, time and therefore costs. If the surface quality desired is high, moreover, a final polish may be required.
EP 0819655 B1 attributes the deviations from the tube geometry in the known methods to disruption to the homogenous temperature field, for example caused by measurement apparatuses in the furnace region or misalignments of the tube longitudinal axis with respect to the axis of symmetry of the temperature field. To remedy this problem, the document describes a method which locally heats or cools the softened glass material in at least one deformation section extending over only part of the periphery of the deformation region as a function of a recorded deviation in the cross-sectional geometry from a desired geometry of the component. The cooling is in this case effected by directing a gas flow onto the glass material. The method serves to eliminate ovality in components with a circular or annular cross section. The gas flow directed onto the surface of the glass material may, however, have adverse effects on the surface quality and therefore the optical properties of optical glass elements. In particular, this can lead to impurities and stresses in the regions close to the surface, which disrupt the beam path. Moreover, it can result in rounding of the edges of glass strands which are polygonal in cross section.
With an optimum axially symmetrical temperature distribution and optimum positioning of the glass strand in the heating apparatus, deformation of glass strands can be avoided only in the case of glass strands which are axially symmetrical in cross section, but not in the case of glass strands which are not axially symmetrical in cross section. The glass particles of a glass strand of any desired cross-sectional or preform geometry also have to move from the outside inward, perpendicular to the drawing direction, on their path through the draw bulb. In the case of a cross section which is not axially symmetrical, not all these flow paths have the same flow resistance. Therefore, in this case of a cross section which is not axially symmetrical, the reduced cross section which results from the acceleration in the drawing direction and the conservation of mass is not identical, or under certain circumstances even not similar, to the original preform geometry even with a homogenous temperature distribution and negligible surface tension effects.